In WDM links protection against light-path failure is typically at a facility level utilizing active optics such as optical switches. For example, as shown in FIG. 1, a double light path connection for a multiple wavelength signal, i.e. a light path connection having a working optical fiber 2 and a redundant optical fiber 4 is connected between optical nodes 6 and 8 in an optical telecommunication system.
At the node 8 of the receive side, the two optical fibers 2 and 4 are combined by an optical switch 10 via which the working optical fiber 2 is connected to the node 8 of the receive side during normal operation, when a demultiplexer 3 demultiplexes the multiple wavelength into separate individual wavelengths. If a break occurs in the working optical fiber 2, which can be identified at the switch 10 on the basis of the outage of the light transmitted over the working optical fiber 2, the switch 10 automatically switches, so that the redundant optical fiber 4 is now connected to the node 8 of the receive side instead of the working optical fiber 2.
At the node 6 of the transmission side a multiplexer 5 multiplexes a plurality of input separate wavelengths into an optical multiple wavelength facility signal to be transmitted, which is split onto the working fiber 2 and the redundant optical fiber 4 by an optical splitter 12. Two optical switches 14 and 16, both of which are closed in normal operation, are now inserted between the optical splitter 12 and the optical fibers 2 and 4. In case of an alternate circuiting, i.e. given a switching at the receive side from the working optical fiber 2 onto the redundant fiber 4, let the node 6 of the transmission side receive a message during the course of a corresponding protocol. In response thereto that optical switch 16 of the two switches 14 and 16, which is inserted between the redundant optical fiber 4 and the optical splitter 12, continues to remain closed. In contrast, optical switch 14, which is inserted between the optical splitter 12 and the working optical fiber 2, is opened.
Thus, by way of the switching at the receive side, the interrupt time associated with the alternate circuiting continues to be kept short, on the one hand, and, on the other hand, it is assured that shortly after the interruption that the broken fiber no longer carries the optical multiple wavelength facility signal.
A problem with WDM system protecting against light path failure at the facility level is that such protection protects only against breaks in the optical fiber carrying the multiple wavelength facility signal, and does not protect against failures on a per channel basis in the optical fiber, and at the respective optical nodes.